Heretofore known techniques for improving handling stability involves reinforcing a rubber with a short fiber such as aramid or cellulose or a crystalline polymer such as syndiotactic polybutadiene to improve its hardness and modulus, for example, its complex elastic modulus (E*) at 70° C. (see e.g., Patent Literature 1). However, even if the elastic modulus is improved, all of the properties other than handling stability, required for tires are not necessarily improved.
Patent Literature 1 proposes a rubber composition formed of a diene rubber component, starch, and cellulose with the aim of providing a rubber composition excellent in abrasion resistance, and also proposes the use of in particular bacterial cellulose as the cellulose. The techniques of Patent Literature 1, however, have the problems of poor breaking properties and large energy loss at the interface between the rubber and the cellulose due to the poor compatibility between the rubber and the cellulose.
Patent Literature 2 discloses a rubber composition that can achieve both low resilience and rigidity (handling stability) and specifically discloses a rubber composition containing a diene rubber together with a fine powder of cellulose fibers prepared from natural plant fibers. The techniques of Patent Literature 2 still have room for improvement in obtaining rigidity and reinforcing properties proportionally to the amount of cellulose fibers added because the cellulose fibers thus prepared have a short fiber length.
When a water dispersion of cellulose is dried, the cellulose aggregates and disadvantageously such cellulose, unless modified, fails to be dispersed at the nano level during mixing with rubber. As a result, it is difficult to improve breaking strength and to reduce rolling resistance. Meanwhile, the conventional treatment for dispersing cellulose includes temporarily dispersing cellulose in water and introducing rubber latex into the dispersion, followed by stirring and drying. This approach is effective for dispersing cellulose, but limits the type of rubber used to latex (see e.g., Patent Literature 3).
Accordingly, it is still difficult to obtain a rubber composition that is environmentally friendly and has excellent breaking properties and low energy loss.